“I lived in Zambia as a child, I got angry seeing blacks being treated like shit.”

Self-confessed “bolshie bastard” Martyn Day and his team of hotshot young lawyers at Leigh Day can congratulate themselves on having won around £150m for tens of thousands of the poorest people on earth from some of the world’s richest companies.

In the past decade, they have challenged Shell, Trafigura, BP, Xstrata, Anglo American and Unilever, as well as the British and Japanese governments. In that time they have carved out a reputation for being the scourge of the corporates and a fierce upholder of human rights.

“We work in the dark, inhospitable corners of the world where miners, oil and mineral companies and governments can often get away with what they could not if they were working in Britain,” says Day. “The multinationals recognise now that if we take them on, we do so seriously. We put the spotlight on them because they should be treating people in places like Bodo in Nigeria in the same way as they would people in Birmingham.”

The former chairman of Greenpeace UK now has 10 teams working in Sierra Leone, Nigeria, Peru, Kenya and elsewhere. The firm will invest millions in a case and take up to 50 lawyers and paralegals to a country to collect witness statements. It has worked for children with leukaemia near Sellafield, former British prisoners of war in Japan and Kenyan people imprisoned during the Mau Mau rebellion.

As one of the very few international law firms prepared to take on the most powerful companies over human rights and the environment, Leigh Day specialises in Africa and developing countries. “Historically, our judiciary used to hate these cases. Now the judges are more internationally aware and interested,” says Day, who runs the international team of around 80 lawyers. “I lived in Zambia as a child. It was the beginning of what I became. I got angry seeing blacks being treated like shit.”

While Day has seen some major coups, wringing £15m last year out of Shell for pollution in the Niger delta and an undisclosed settlement for 30,000 Ivorians affected by chemical pollution, the firm has lost major cases, too.

The most embarrassing was the al-Sweady inquiry in 2014, when Leigh Day represented nine Iraqi soldiers who claimed that the UK army had tortured and murdered detainees following the Battle of Danny Boy in 2004. But the case fell apart with the disclosure of a letter showing that some of the Iraqis were part of the Mahdi army. The judge concluded that the most serious allegations were “deliberate lies, reckless speculation and ingrained hostility”.

Day says: “Maybe the Iraqis were fantastic liars. It seems they hoodwinked us. We were criticised by the minister in parliament. Perhaps I am a shit lawyer, but I stand by the fact that [the allegations] warranted an inquiry. If you put your head above the parapet, they will shoot. We do high-profile cases. We make big enemies. When they get a chance, they have a go at us.”

Day started with a trade union law firm, from which he was sacked “for causing trouble”, and went on to London law firm Bindmans, where he went on strike for a day over what he thought was the clerks’ low pay. “I was a bolshie bastard. My skill in life is as a troublemaker,” he says.

But the firm has grown by 20% a year for a decade, now has 400 lawyers and 34 partners and is Britain’s largest firm of criminal neglect lawyers.

Day now receives almost one request each day to investigate a potential corporate or government wrongdoing. “We attract driven young lawyers. We have fabulous contacts with NGOs. I need people who are driven,” he says.

“We do big and unusual cases. We want to act for people who have serious problems.”